The rumor came from Summer herself, when she heard about the movie getting greenlighted she made a remark in an interview she did about having a dream about Joss recasting the part of River for the movie. However, as she said Joss wasn't going to do the movie without the original cast.

What I heard was that Summer went to Joss after hearing about the greenlight and told him that if he had to recast River with some big-name actress, she'd understand. To which Joss said, no way in the 'Verse.

I love Adam's comment that you always play the hero- it reflects what Joss said in the commentary for Serenity (Series movie) about the interrogation scene with Lawrence and Jayne. Jayne always saw himself as the hero of the show.

--Rob Schneider/Wash: It looks like the frickin' thing just fell off the ship! Sex with a dolphin!

*pause*

Keanu Reeves/Mal: *deer in headlights* ...There is no spoon?

--I also now have an image of Sylvester Stallone in Jayne garb screaming, "Veeeeeeeeeeeeera!" as Kevin Federline/Simon cuts his eye open so he can fight Carl Weathers/Operative. 'Cuz we all know that Carl Weathers needs to be the Operative.

And it certainly means Kaylee will be knocked up in the sequel, Serenity 2: Rocky Fully Reloaded at the Crossroads Having Good Times in a House of Wax with the Demolition Man, featuring Carmen Electra. ;-)